With this software you can investigate how sound waves travel through the air and how shock waves are formed. As any object moves through the air, the air near the object is disturbed. The disturbances are transmitted through the air at a distinct speed called the speed of sound, because sound itself is just a sensation created in the human brain in response to small pressure fluctuations in the air. In our simulation, a bug is creating a sound which moves through the air as a series of waves. When the waves pass our microphone, a sound is detected. The distance between any two waves is called the wavelength and the time interval between waves passing is called the frequency . The brain associates a certain musical pitch with each frequency; the higher the frequency, the higher the pitch. Likewise, shorter wavelengths produce higher pitches. The speed of transmission of the sound remains a constant regardless of the frequency or the wavelength. The speed of sound only depends on the state of the air (or gas) medium not on the characteristics of the generating source.
Because the speed of sound depends only on the state of the gas, some interesting physical phenomena occur when a sound source moves through a uniform gas. You can study some of these phenomena by setting the bug in motion by using the slider at the bottom of the program. If the source moves slower than the speed of sound, conditions are said to be subsonic. As the source moves it continues to generate sound waves which move at the speed of sound. Since the source is moving slower than the speed of sound, the waves move out away from the source. Upstream (in the direction of the motion), the waves bunch up and the wavelength decreases. Downstream, the waves spread out and the wavelength increases. The sound that our microphone detects will change in pitch as the object passes. This change in pitch is called a doppler effect. If the source moves at or near the speed of sound conditions are said to be sonic or transonic. In this case, the waves again bunch up upstream and spread out downstream. But because the source speed is nearly the sound speed, the upstream wavelength becomes nearly zero and the individual waves collect into a single shock wave. Transonically, the shock wave is nearly perpendicular to the flow direction and across the shock wave the conditions of the flow change abruptly. The microphone now detects a sharp sonic boom associated with the passing of the shock wave, and then detects the source moving away. If the source moves higher than the speed of sound conditions are said to be supersonic. The object generates a conical shock wave and the microphone again detects a sonic boom when the shock passes by. The angle of the cone depends on the relation between the speed of sound and the speed of the object; the faster the speed of the object the sharper (smaller) the cone angle. Notice that the source can move far downstream of the microphone before the boom is detected.
As we have seen, the ratio of the speed of the source to the speed of sound determines whether we have shock waves present in the flow or not. Because of the importance of this speed ratio, aerodynamicists have designated it with a special parameter called the Mach number. The Mach number is named in honor of Ernst Mach, a late 19th century physicist who studied gas dynamics. Subsonic conditions occur for Mach numbers less than one, transonic conditions occur near Mach equals one, and supersonic conditions occur for Mach numbers greater than one.
With this simulator, you can set the speed of the source with the slider. You can start or stop the motion of the bug by using the appropriate buttons. If the source is stopped, you can advance or retreat by a single time increment by using the "Step Forward" and "Step Back" buttons. And you can "Resume" the animation by pushing that button. Notice how the frequency of the sound changes when the source is moving and notice how the waves collect into a shock wave when the object moves transonically or supersonically.
You can download your own copy of this simulator for use off line. The program is provided as Sound.zip. You must save this file on your hard drive and "Extract" the necessary files from Sound.zip. Click on "Mach.html" to launch your browser and load the program.
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